Semiaktive

Semiaktive (semi-active) systems are control mechanisms in engineering that adjust their properties in real time without delivering net energy to the controlled process. They sit between passive systems, which have fixed properties, and active systems, which inject energy to drive motion. Semi-active devices reduce disturbances by changing how energy is exchanged with the environment rather than by supplying power to the motion itself.

In practice, semi-active devices modify dissipation or stiffness in real time through controllable actuators. The most common implementations are semi-active dampers, such as magnetorheological (MR) or electrorheological (ER) dampers, whose damping coefficients change in response to applied magnetic or electric fields. Other approaches include controllable fluid dampers and variable-damping springs. The energy required to adjust these devices is typically small compared to the energy of the system being controlled.

Control strategies rely on sensors and algorithms to select settings that reduce vibrations or transient responses. Popular approaches include clipped-optimal control, skyhook-like logic, and fuzzy control. The goal is to achieve noticeable vibration reduction with low power consumption and high robustness, without the complexity of fully active systems.

Applications of semi-active technology span automotive suspensions, civil and aerospace structures for vibration mitigation, and precision machinery to reduce chatter. Benefits over passive solutions include adaptivity and improved performance with lower energy requirements than active systems. Limitations include the inability to inject net energy to fully cancel disturbances, so effectiveness depends on the disturbance profile and the range of adjustable damping or stiffness. The term Semiaktive is commonly used in German-language literature to denote these semi-active control concepts.